The field of the invention is acoustical imaging and, in particular, ultrasound imaging using reflection mode computerized tomography.
There are a number of modes in which ultrasound can be used to produce images of objects. The ultrasound transmitter may be placed on one side of the object and the sound transmitted through the object to the ultrasound receiver which is placed on the other side ("transmission mode"). With transmission mode methods, an image may be produced in which the brightness of each pixel is a function of the amplitude of the ultrasound that reaches the receiver ("attenuation" mode), or the brightness of each pixel is a function of the time required for the sound to reach the receiver ("time-of-flight" or "speed of sound" mode). In the alternative, the receiver may be positioned on the same side of the object as the transmitter and an image may be produced in which the brightness of each pixel is a function of the amplitude or time-of-flight of the ultrasound which is reflected from the object back to the receiver ("refraction", "backscatter" or "echo" mode). The present invention is a backscatter method for producing ultrasound images.
There are a number of well known backscatter methods for acquiring ultrasound data. In the so-called "A-scan" method, an ultrasonic pulse is directed into the object by the transducer and the amplitude of the reflected sound is recorded over a period of time. The amplitude of the echo signal is proportional to the scattering strength of the refractors in the object and the time delay is proportional to the range of the refractors from the transducer. In the so-called "B-scan" method, the transducer transmits a series of ultrasonic pulses as it is scanned across the object along a single axis of motion. The resulting echo signals are recorded as with the A-scan method and either their amplitude or time delay is used to modulate the brightness of pixels on a display. With the B-scan method, enough data are acquired from which an image of the refractors can be reconstructed.
In the so-called C-scan method, the transducer is scanned across a plane above the object and only the echoes reflecting from the focal depth of the transducer are recorded. The sweep of the electron beam of a CRT display is synchronized to the scanning of the transducer so that the x and y coordinates of the transducer correspond to the x and y coordinates of the image.
Computer tomography has found widespread use in the medical field since its discovery by Hounsfield in 1973. Tomography is commonly implemented by revolving an X-ray source and an opposing X-ray detector about the patient. A series of measurement are made from different angles as the source and detector revolve about the patient. The resulting X-ray absorption data are used to reconstruct an image which is a cross sectional view through a single plane. The image is typically reconstructed using a Fourier transform or filtered back propogation method. Although transmission mode ultrasound tomography has been successfully implemented, refraction mode ultrasound tomography has not been successful due to variations in the speed of sound as the transmitter and receiver are revolved around the patient.